Allow interactions to deal directly with browser events

The `relayEvent_` method wraps goog's browser events in our `ol.MapBrowserEvent`.  This allows interactions to deal with real browser events in addition to our emulated events.

Though the `click` and `mousemove` events are not currently used by interactions in master, they are used by interactions in the edit branch (the emulated `singleclick` is not usable for drawing).  I'd very much like to get small pieces of the edit work reviewed and merged.  The merge conflicts with the event handling in particular is getting tedious.